Allow passing in an existing ArgParser for a command.

pkg/args/test/command_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library command_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:args/args.dart';
 
 main() {
   group('ArgParser.addCommand()', () {
+    test('creates a new ArgParser if none is given', () {
+      var parser = new ArgParser();
+      var command = parser.addCommand('install');
+      expect(parser.commands['install'], equals(command));
+      expect(command is ArgParser, isTrue);
+    });
+
+    test('uses the command parser if given one', () {
+      var parser = new ArgParser();
+      var command = new ArgParser();
+      var result = parser.addCommand('install', command);
+      expect(parser.commands['install'], equals(command));
+      expect(result, equals(command));
+    });
+
     test('throws on a duplicate command name', () {
       var parser = new ArgParser();
       parser.addCommand('install');
       throwsIllegalArg(() => parser.addCommand('install'));
     });
   });
 
   group('ArgParser.parse()', () {
     test('parses a command', () {
       var parser = new ArgParser();
@@ skipping 168 matching lines @@
   });
 }
 
 throwsIllegalArg(function) {
   expect(function, throwsArgumentError);
 }
 
 throwsFormat(ArgParser parser, List<String> args) {
   expect(() => parser.parse(args), throwsFormatException);
 }